Medical devices may be coated so that the surfaces of such devices have desired properties or effects. For example, it may be useful to coat medical devices to provide for the localized delivery of therapeutic agents to target locations within the body, such as to treat localized disease (e.g., heart disease) or occluded body lumens. Localized drug delivery may avoid some of the problems of systemic drug administration, which may be accompanied by unwanted effects on parts of the body which are not to be treated. Additionally, treatment of the afflicted part of the body may require a high concentration of therapeutic agent that may not be achievable by systemic administration. Localized drug delivery may be achieved, for example, by coating balloon catheters, stents and the like with the therapeutic agent to be locally delivered. The coating on medical devices may provide for controlled release, which may include long-term or sustained release, of a bioactive material.
Aside from facilitating localized drug delivery, medical devices may be coated with materials to provide beneficial surface properties. For example, medical devices are often coated with radio-opaque materials to allow for fluoroscopic visualization while placed in the body. It is also useful to coat certain devices to achieve enhanced biocompatibility and to improve surface properties such as lubriciousness.
During deployment and loading of self-expanding (SE) stents, there may be significant friction between the stent surface and the sheath. Longer stents may have higher friction forces. These shear forces may be especially damaging in relation to coated SE stents. As the application of drug eluting (DE) coatings to progressively longer stents occurs, the problems resulting from this frictional interaction may increase.
Self-expanding stents with drug-eluting coatings are being developed in increasing lengths, up to 150 mm and longer. A DE SE stent loaded in a delivery catheter may apply a compressive force against an inside surface of the delivery catheter. This compressive force may be supported directly by the coating, which may consist of a thin, relatively soft polymer carrier of the bioactive substance. During stent deployment, the compressive force combined with linear displacement may produce an abrasive, scraping action against the DE coating. This friction may also be responsible for increasing the stent deployment force, which may be increased to levels higher than are considered acceptable. These related problems may be progressively exacerbated as stent lengths increase. Additionally, a stent deployed a few minutes after being loaded may exhibit a lower stent deployment force compared with one deployed several months after being loaded.
Catheters have been reinforced with fine wire braid to increase their hoop stress, (to increase indentation resistance), and lined with thin coatings of low-friction materials such as PTFE (polytetrafluoroethylene), or alternatively, ePTFE (expanded polytetrafluoroethylene). These efforts may be problematic, particularly for the longest stents.
Deployment systems for protecting DE coatings include a rolling sheath or membrane. Although feasible, rolling sheaths or membranes may require quite difficult processes to produce and assemble the rolling membrane into the finished delivery system. At the rolling end, the membrane may turn inside out on itself and cause a load to be added to the retraction force as the outer portion of the membrane is pulled over the inner portion.
Stents with controlled expansion are apparently discussed in U.S. Pat. No. 6,613,077 to Gilligan et al., entitled “Stent with Controlled Expansion”. An activation mechanism for a catheter is discussed in U.S. Pat. No. 6,391,051 to Sullivan, III et al., entitled “Pull Back Stent Delivery System with Pistol Grip Retraction Handle”.
There is therefore a need for reducing deployment forces and protecting DE coatings on SE stents, in particular longer SE stents.
Each of the references cited herein is incorporated by reference herein for background information.